Runaway merger shocks in galaxy cluster outskirts and radio relics

Abstract

Moderately strong shocks arise naturally when two subclusters merge. For instance, when a smaller subcluster falls into the gravitational potential of a more massive cluster, a bow shock is formed and moves together with the subcluster. After pericenter passage, however, the subcluster is decelerated by the gravity of the main cluster, while the shock continues moving away from the cluster center. These shocks are considered as promising candidates for powering radio relics found in many clusters. The aim of this paper is to explore the fate of such shocks when they travel to the cluster outskirts, far from the place where the shocks were initiated. In a uniform medium, such a "runaway" shock should weaken with distance. However, as shocks move to large radii in galaxy clusters, the shock is moving down a steep density gradient that helps the shock to maintain its strength over a large distance. Observations and numerical simulations show that, beyond R 500 , gas density profiles are as steep as, or steeper than, ∼ r −3 , suggesting that there exists a "Habitable zone" for moderately strong shocks in cluster outskirts where the shock strength can be maintained or even amplified. A characteristic feature of runaway shocks is that the strong compression, relative to the initial state, is confined to a narrow region just behind the shock. Therefore, if such a shock runs over a region with a pre-existing population of relativistic particles, then the boost in radio emissivity, due to pure adiabatic compression, will also be confined to a narrow radial shell.1 The boundary separating the gaseous atmosphere of the main cluster from that of the subcluster is a contact discontinuity, a.k.a. cold front.

ClG 0217+70: A massive merging galaxy cluster with a large radio halo and relics

“…1). This cluster may thus provide a good example of runaway merging shocks (Zhang et al 2019), which are long-lived in the habitable zone in the cluster outskirts.…”

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA

“…Firstly, we outline the broad brush scenario, originally mentioned in Burns et al (1994) and Biviano et al (1996) and discussed in detail in Lyskova et al (2019); Sheardown et al (2019); Zhang et al (2019). In this scenario, the NGC 4839 group was initially moving from the NE along the general direction of the Coma-A1367 filament.…”

“…Left: flattened image of the Coma cluster field with labels schematically marking some of the features associated with the merger with the NGC4839 group. The blue dashed line is the suggested trajectory of the group (Lyskova et al 2019;Sheardown et al 2019;Zhang et al 2019), which enters the Coma cluster from NE, and is currently close to the apocenter. The presumed position of the primary shock is shown with the red curve so that it goes through the radio relic SW from the NGC4839 group.…”

“…However, downstream of a spherical merger shock (the primary shock), the compressed region is followed by a rarefaction, where the density falls below the initial density of a given gas parcel (see Fig. 5 in Zhang et al 2019). This rarefaction is caused by the expansion of the gas that is pushed by the shock to larger radii.…”

“…The outer one (radio relic), located close to the position of the primary shock -is formed due to particle acceleration and/or compression at the shock, followed by adiabatic expansion and radiative losses A41, page 12 of 16 at high frequencies (see, e.g., Fig. 7 in Zhang et al 2019). The inner one, inside the mini-accretion shock, is a more diffuse and extended radio halo.…”